Turns out the border wall, when it spans a river (like the Rio Grande), is violating a 48 year old treaty with Mexico. The treaty says neither side can obstruct the waterway unless both sides agree. The big issue at hand is areas of Mexico being subject to increased flooding. If things progress, there is a chance it would end up in international court which would probably mean the treaty is to be honored. (None of that is to say that 45 wouldn’t just do what he wants and risk trade war or real war or something).